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Decisions ripple through flocks of birds like a wave

By Jacob Aron

Video: Watch a decision spread like a wave

Starlings don’t get into a flap, they go with the flow

(Image: Filippo Monteforte/AFP/Getty Images)

Birds of a feather flock together, but perhaps not in the way we thought. An analysis of flocking starlings suggests the decision to turn spreads through the birds like a rippling wave, rather than diffusing through the group like a gas.

Asja Jelić of the Sapienza University of Rome, Italy, and her colleagues spend their nights watching starlings fly over the city.

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“During winter they come and do this flocking phenomenon every evening,” she says. The birds move in near-unison, almost as if they were a single organism, prompting researchers to wonder how each bird knows when to turn.

Current theories hold that flocking animals like starlings attempt to stay a certain distance apart at all times. Any decision to turn would therefore spread slowly through the group in multiple directions, like a gas in a box.

But observations from Jelić and her team suggest otherwise. The team uses a system of three cameras to record the birds so they can analyse their movements. An earlier version of this setup could pinpoint the birds in 3D space, but had trouble tracking individuals because starlings are identical. The latest upgrade can also track an individual bird’s movement over time.

On the wing

By comparing at what point each bird starts turning, they discovered that there are always a few birds flying close together that initiate the turn, although these aren’t always the same birds – there is no official leader. The decision to turn then propagates from bird to bird like a wave, at a speed that depends on how parallel the birds’ paths are.

Learning how information flows through groups could be useful in other fields, such as robotics and financial markets, says Jelić. “Flocking is just one kind of collective behaviour.”

Christos Ioannou at the University of Bristol, UK, says the next stage is to see what happens to the spread of information when you add a predator to the mix. “You don’t know if it is ecologically relevant information,” he says. “If the turns are being caused by a falcon, are those turns different?”